Hydraulic fluids are used to transfer energy through fluid conduits to cause a force to be exerted against a moveable object (e.g. a piston) resulting in work. In order to function efficiently, it is necessary that such fluids be stable and preferably fire resistant. A very important property is that the fluid have a low pour point and that it does not form any solid precipitate when maintained at low temperature for an extended period. Solids could plug orifices and cause a malfunction in the hydraulic system.
Because of their fire resistance, polyphosphazenes have been investigated for some time in an attempt to prepare a suitable hydraulic fluid. Kober et al., U.S. Pat. No. 3,291,865 describes the preparation of hydraulic fluids by reacting mixtures of phenols or substituted phenols and fluoroalcohols with cyclic phosphonitrilic chloride trimer.
Lederle et al., J. Chem. & Eng. Data, Vol. 11, No. 2, April 1966, page 221 describes fire resistant hydraulic fluids made in a manner similar to Kober et al. Ottmann et al., Ind. & Eng. (Chem. Prod. Res. & Dev.), Vol. 5, No. 2, June 1966, page 202 describe arylamino polyfluoroalkoxy-substituted phosphonitrilic cyclic trimers made by the addition of N-methylaniline and triethylamine to a stoichiometric excess of phosphonitrilic chloride trimer. This in turn is then added to a solution of sodium fluoroalkoxide to complete the reaction.
Singler et al., "Army Science Conference Paper", A117298 describe the synthesis and evaluation of phosphazene fluid in an attempt to find a replacement for triarylphosphates under military specification MIL-H-19457c. This research involved the use of trifluoroethoxy-substituted cyclic phosphonitrilates which were also substituted with either m-chlorophenoxy or m-trifluoromethylphenoxy groups. Three modes of synthesis are described:
(1) addition of trimer to a mixture of sodium aryloxide and sodium trifluoroethoxide, PA0 (2) addition of a mixture of sodium aryloxide and trifluoroethoxide to trimer, and PA0 (3) sequential addition of sodium aryloxide followed by sodium trifluoroethoxide to trimer.
Crystallization appears to have been encountered in all cases but oil recovery was increased by following the methods of modes two or three.
Singler, "Potential of Phosphazenes as Hydraulic Fluids", Hydraulic Fluids Meeting, NASA Ames Research Center, February 1976 summarizes the effect of various substituents on the substituted cyclic phosphonitrilate trimers and tetramers.
More recently, Singler et al., Ind. Eng. Chem. (Prod. Res. & Dev.), Vol. 25, No. 1 (1986) describe hydraulic fluids made by substituting cyclic phosphonitrilic chloride with both aryloxy groups and trifluoroethoxy groups. Reference is again made to the three modes of synthesis; "which yielded widely different ratios of solids to fluids". For example, mode two in which an equal mole mixture of sodium m-chlorophenate and sodium trifluoroethoxide was added to trimer increased the oil recovery from 18% up to 62%.
Carr, Eur. Pat. Appl. No. 0 145 002, published June 19, 1985 discloses a phosphazene fluid prepared by reacting cyclic phosphonitrilic chloride trimer with a phenol, a polyfluoroalcohol or mixtures thereof in a two-phase medium comprising water and a water-immiscible solvent. The reaction is promoted by use of a base, e.g. sodium hydroxide, and a phase transfer catalyst. In Example 4, the process carried out with trifluoroethanol and phenol is said to form an almost colorless clear liquid.